1. Field of the Invention
The present invention relates to a high-voltage power supply apparatus. More specifically, the present invention relates to a high-voltage power supply apparatus which controls supply voltages to two circuits respectively requiring voltages of different magnitudes which are both connected to an output winding of a transformer.
2. Description of the Prior Art
Conventionally, equipment requiring a relatively high medium voltage, and also a voltage that is higher than the medium voltage (hereinafter referred to as "medium voltage" and "high voltage"), for example, an electrostatic copier, electrostatic printer, electrostatic facsimile or the like, is provided with individual transformers dedicated to the medium voltage and the high voltage, respectively.
Such installation of the dedicated transformers in the respective circuit requiring the medium voltage (hereinafter referred to as "medium voltage circuit") and circuit requiring the high voltage (hereinafter referred to as "high voltage circuit") not only makes miniaturization of the power supply apparatus difficult but also raises the cost thereof.
On the other hand, power may be simply supplied simultaneously to both the medium voltage circuit and the high voltage circuit using a common transformer as shown in FIG. 1.
FIG. 1 is a circuit diagram showing an example of a conventional high-voltage power supply apparatus which is in the background of the present invention. In this conventional circuit, a medium voltage circuit 2 and a high voltage circuit 3 are commonly connected to an output side of an output winding 1a of a transformer 1 through a diode. In the case where this conventional circuit is used for the electrostatic copier, the medium voltage circuit 2 is, for example, a bias developing circuit and, the high voltage circuit 3 includes, for example, a corona discharger. When a voltage of the output side of the output winding is higher than a voltage required by the corona discharger or high voltage circuit 3, both the medium voltage circuit 2 and the high voltage circuit 3 are simultaneously operated. However, when the output voltage becomes lower than the corona discharge voltage, the high voltage circuit 3 stops operating.
In the conventional circuit as shown in FIG. 1, only a current through the high voltage circuit 3 flows through the detection resistor Rf when the high voltage circuit 3 is operated. Thus, by sensing the voltage across R.sub.f, an input voltage of the transformer is able to be controlled by this current. However, since no current flows through the detection resistor Rf when the high voltage circuit 3 is turned off, it is impossible to control the input voltage at this time, when the current flows only through the medium voltage circuit 2.